Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081 PrimeCells

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, Mar 10, 2014 at 06:56:30AM -0700, David Woodhouse wrote:
> On Sat, 2011-01-01 at 15:15 +0000, Russell King - ARM Linux wrote:
> > On Wed, Dec 22, 2010 at 03:45:39PM -0800, Dan Williams wrote:
> > > Support for the DMA_COMPL flags are necessary if the DMA_MEMCPY
> > > capability is advertised, yes this driver got this wrong.  I'll update
> > > the documentation to make this requirement clear, and audit the other
> > > drivers.  With slave-only drivers the only usage model is one where
> > > the client driver owns dma-mapping.  In the non-slave (opportunistic
> > > memcpy offload) case the client is unaware of the engine so the driver
> > > owns unmapping.  The minimal fix is to disable memcpy offload.
> > 
> > As a side note, the DMA mapping for slaves should be done using the
> > DMA struct device, not the struct device of the peripheral making use
> > of the DMA engine.
> > 
> > Why?  The slave device has no knowledge of how the DMA engine is
> > connected into the system, or the DMA parameters associated with the
> > device performing the DMA, such as the DMA mask and boundaries.  (If
> > there are several generic DMA agents in the system, it can't know
> > which is the correct one to use until a channel has been allocated.)
> > The only struct device which has this information is the one for the
> > DMA engine itself.
> > 
> > Therefore, the struct device which is passed into the DMA mapping APIs
> > to prepare memory for DMA must always be the DMA engine struct device
> > (chan->device->dev) and never the slave struct device.
> 
> That all seems eminently sensible. However, I wonder if it always has to
> be true.
> 
> It is not impossible for the DMA controller to "delegate" transactions
> so that (to the IOMMU) they appear to come from the individual slave
> device rather than from itself.
> 
> The Intel IOMMU has now gained support for DMA mapping for devices
> enumerated by ACPI — essentially the ACPI "DMAR" table just has a lookup
> table of ACPI device paths, and tells us the PCI bus/devfn that their
> DMA transactions will *appear* to be from.
> 
> What we've seen is that it is the individual slave devices that are
> listed in these tables, *not* the DMA controller itself. It looks like
> we are actually expected to set up the IOMMU mapping for the *slave*,
> not the DMA controller. The system doesn't even *tell* me how to set up
> DMA mappings for the DMA controller device; only the slaves.

Okay, so how do you get the DMA address which is to be programmed into
the DMA controller - bearing in mind that different devices in the
system may have different bus:physical offsets?

ACPI may allow you to work this out for each slave device, but now
try thinking about this same problem without ACPI.

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